(1) Field of the Invention
The present invention relates to an antenna system and particularly, but not exclusively, to a phased array antenna system having a plurality of antenna elements arranged in at least two sub-arrays. The antenna system is suitable for use in many telecommunications systems but finds particular application in cellular mobile radio networks, commonly referred to as mobile telephone networks. More specifically, the antenna system of the present invention may be used with third generation (3G) mobile telephone networks and the Universal Mobile Telephone System (UMTS).
(2) Description of the Art
Operators of cellular mobile radio networks generally employ their own base-stations, each of which includes one or more antennas. In a cellular mobile radio network, the antennas are a factor in defining the desired coverage area which is generally divided into a number of overlapping cells, each associated with a respective antenna and base station. Each cell contains a fixed-location base station which communicates with mobile radios in that cell. The base stations themselves are interconnected by other means of communication, either radio links or fixed land-lines, and are arranged in a grid or meshed structure allowing mobile radios throughout the cell coverage area to communicate with each other as well as with the public telephone network outside the cellular mobile radio network.
The antennas used in such networks are often composite devices known as phased array antennas which comprise a plurality (usually eight or more) or array of individual antenna elements or dipoles. The direction of maximum sensitivity of the antenna, i.e. the vertical or horizontal direction of the main radiation beam or “boresight” of the antenna pattern, can be altered by adjusting the phase relationship between the elements. This has the effect of allowing the beam to be steered to modify the coverage area of the antenna.
In particular, operators of phased array antennas in cellular mobile radio networks have a requirement to adjust the vertical radiation pattern (VRP), also known as the “tilt”, of the antenna since this has a significant effect on the coverage area of the antenna. Adjustment of the coverage area may be required, for example, owing to changes in the network structure or the addition or removal of other base stations or antennas in the cell.
The adjustment of the angle of tilt of an antenna is known and is conventionally achieved by mechanical means, electrical means, or both, within the antenna itself. When tilt is adjusted mechanically, for example by mechanically moving the antenna elements themselves or by mechanically moving the housing for the elements, such an adjustment is often referred to as “adjustment of the angle of mechanical tilt”. The effect of adjusting the angle of mechanical tilt is to reposition the boresight such that it points either above or below the horizon. When tilt is adjusted electrically, by adjusting the phase of signals supplied to the antenna elements without physically moving either the housing for the elements, the antenna elements themselves or any other part of the antenna radome, such an adjustment is commonly referred to as “adjustment of the angle of electrical tilt”. The effect of adjusting the angle of electrical tilt is also to reposition the boresight so that it points either above or below the horizon but, in this case, is achieved by changing the time delay of signals fed to each element (or group of elements) in the array.
A disadvantage of mechanical adjustment of the angle of electrical tilt is that it must be carried out in situ by manual mechanical adjustment of the antenna.